Your concerns are misplaced. 5% citric acid (7 ounces added to a gallon of water; a lot more than Magnum Wheel Man is using) is an old Frankford Arsenal case cleaning formula from before the military found cases with oxides left on them had better corrosion resistance than cleaned and polished ones do. The reaction of the citric acid with brass tends to be self-limiting for practical purposes. That is, it reacts far, far more vigorously with oxides than with metallic brass. You can leave a case in it a very long time before thinning it to any significant degree.

Citric acid is also is used as a water softener, as it helps tie up minerals in hard water. That further improves cleaning properties of the water.

Dishwashing liquids and detergents that are labeled "mild" are not alkaline like soaps because they are not dissolved fats the require hydroxide ions to make them soluble in water. They tend to be somewhat alkaline as manufactured, but often the pH is brought down to make them easier on the hands. For example, in baby shampoo and conditioning shampoos, because being alkaline would dissolve or damage the oils they put in to condition the hair, the pH needs to come down to 7 or even go slightly acid. How, you may ask, do they bring the pH down? They add citric acid, as it not only adjusts the pH down, but it also is a preservative antioxidant for the oils. Yet the shampoos still clean.

Below are case cleaned in an ultrasonic for half an hour in a 140°F 5% solution of citric acid with Dawn PE mixed in. The oxides were gone in the first minute. The carbon took longer. Measurements of the necks with a tubing micrometer show now measurable loss of thickness. The head stamp is sharp and internal flash hole drilling burrs were still intact (unfortunately), indicating very little metal removal.

Though the cases in the bottom picture are not in the same order they were at the top, you can still see that it is the oxidized location that are left pink, as the zinc bore the brunt of the reaction with oxygen.

Before:

After:

All of these cases proved to be usable, by the way. The worst corrosion pits were only a couple thousandths deep, so no more than 10% of wall thickness on the thinnest part of the sides.